Isaac lashing the Lesser Antilles; TD 10 forms

Tropical Storm Isaac is lashing the entire Lesser Antilles chain of islands with heavy rains this morning, and the winds will be on the increase this afternoon as the storm heads west at 19 mph. Isaac is still a weak and disorganized tropical storm, but that is changing. The Hurricane Hunters are in the storm, and have thus far found no increase in the storm's top surface winds, which remain near 45 mph. In their 7:44 am EDT center fix, an Air Force Reserve aircraft measured a rather unimpressive center pressure of 1007 mb, and top winds at their 1000 foot flight altitude of 56 mph. The plane did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. A large area of dry air to the north of the storm, as seen on water vapor satellite loops, continues to interfere with development, and it is unlikely Isaac will become a hurricane today. Satellite loops, however, show that Isaac has developed a Central Dense Overcast (CDO) of high cirrus clouds, the hallmark of a developing storm. These clouds have very cold cloud tops, indicating that the updrafts creating them are quite strong. An analysis of upper level winds from the University of Wisconsin CIMSS shows that upper-level outflow is becoming well-established to the southwest and northwest, but outflow is restricted on the southeast side. A large clump of heavy thunderstorms several hundred miles southeast of the center continues to compete for moisture, and is interfering with the low level inflow and upper level outflow of the storm. The intensification rate of Isaac will increase if the storm is able to integrate this clump of heavy thunderstorms into its circulation, which satellite loops this morning suggest is now beginning to happen. Radar imagery from Barbados and Martinique show plenty of heavy rain showers, mostly on the south side of Isaac where it is moister, but little spiral banding. Hurricane hunter missions are scheduled for Isaac every six hours, and a NOAA hurricane hunter research aircraft will also be in the storm, with missions scheduled every 12 hours. The NOAA jet is first scheduled to fly into the storm on Thursday afternoon, to do a large-scale dropsonde mission to aid model forecasts.

Intensity forecast for IsaacThe latest 8 am EDT run of the SHIPS model shows that wind shear is moderate, 10 - 15 knots, but is expected to relax by this evening to a low 5 - 10 knots. Ocean temperatures have increased to a very warm 29°C, and the storm is now over waters with a high total heat content. The lowering wind shear and warm waters should allow the storm to wall off the dry air that has been interfering with development, and also allow the storm to integrate the thunderstorm clump on its southeast side that has been interfering with low level inflow and upper level outflow. It will take some time for the increase in organization to result in an increase in Isaac's winds, and I still expect top winds of 45 - 60 mph in the Lesser Antilles Islands this evening when the core of the storm moves through. On Thursday, when Isaac will be in the Eastern Caribbean, conditions should be favorable enough to allow steady strengthening to a Category 1 hurricane. The 11 am EDT wind probability forecast from NHC predicted a 47% chance that Isaac will become a hurricane by Friday morning, and a 16% chance it will be a Category 2 or stronger hurricane then. This is a reduction in the odds given in the 5 am advisory.

Impact of Isaac on the IslandsThe entire Lesser Antilles Islands chain will have a three-day period of heavy weather Wednesday through Friday. Sustained tropical storm-force winds extend out about 50 miles to the north of the center and 30 miles to the south, so an 80-mile wide swath of the Lesser Antilles will potentially see tropical storm-force winds of 45 - 60 mph this Wednesday evening. Guadaloupe, Antigua and Barbuda, Dominica, Montserrat, and St. Kitts and Nevis at highest risk of these winds.

Winds in St. Croix in the Virgin Islands will likely rise above tropical storm force on Thursday morning, and the south coast of Puerto Rico should see tropical storm-force winds by Thursday afternoon. The San Juan airport may be able to stay open Thursday afternoon and evening, but I think it is more likely they will be forced to shut down.

On Thursday night, heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic, and all airports in the D.R. will probably be closed on Friday. Rainfall amounts of 8 - 12 inches will likely affect the Dominican Republic Thursday through Saturday, creating dangerous flash floods and mudslides.

Isaac is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from Isaac will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 8 - 12 inches are possible, which will be capable of causing extreme flooding on the vegetation-denuded slopes of Haiti. It will be a major challenge to keep those Haitians living outside safe, if rainfall amounts of 5 - 10 inches occur.

Impact on Florida, Cuba, Jamaica, and the BahamasFlorida, Cuba, Jamaica, and the Bahamas are all at high risk of receiving hurricane conditions from Isaac. The latest set of 06Z (2 am EDT) model runs for Isaac are fairly unified for the coming three days, showing a westward track to a point on the south coast of Hispaniola. All of the models then predict a more west-northwest track across the island and into eastern Cuba, as Isaac responds to a trough of low pressure over the Southeast U.S. Most of the models then predict a path for Isaac along the spine of Cuba, then into the Florida Straits off the coast of Miami by five days from now. A notable exception is our best-performing model, the ECMWF, which keeps Isaac just south of Cuba, and takes the storm more to the west between Jamaica and Cuba on Saturday, then into the Yucatan Channel between Mexico and Cuba by Monday. However, this model is keeping Isaac weaker than the other models, and thus predicts the storm will have a weaker response to the trough of low pressure over the Southeast U.S. If the official NHC intensity forecast is right and Isaac becomes a hurricane on Thursday, the more southerly track of the ECMWF is not going to verify, and Isaac will spend considerable time over Cuba on Saturday and Sunday. Where Isaac pops off the coast of Cuba will be critical in determining its future path and intensity. Some models predict a more easterly exit point, allowing Isaac to move up the east coast of Florida, and potentially make landfall in the Southeast U.S. The latest 06Z GFS model run predicts a more westerly track, which would potentially allow Isaac to move up the west coast of Florida towards Tampa. Keep in mind that the average error in a 5-day forecast is 260 miles. The two most recent runs of the GFS model, at 00Z and 06Z (8 pm and 2 am EDT), gave positions for Isaac that were about 250 miles apart--the earlier run putting the center near West Palm Beach, and the more recent run giving a location between Key West and Havana, Cuba. While passage over the high mountains of Hispaniola and then Cuba will substantially disrupt Isaac and probably reduce it below hurricane strength, the storm is quite large, and should be able to re-intensify once it emerges over the Florida Straits. Waters will be very warm, near 30°C, wind shear is predicted to be light, and forecasts of the upper-level winds show the possibility of an upper-level outflow pattern very favorable for intensification. If Isaac spends a day over water, that should be enough time for it to intensify into a Category 1 hurricane, and if the storm takes a longer 2-day track over water up either the east or west coast of Florida, a Category 2 or stronger storm is possible.

Isaac is a threat to affect Tampa during the Republican National Convention, August 27 - 30, and the official NHC forecast now has Tampa in the 5-day cone of uncertainly. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 9% chance of receiving tropical storm-force winds for the 24-hour period ending on the morning of first day of the convention (Monday). I blogged about the climatological chances of a hurricane causing an evacuation of Tampa during the convention in a post last week, putting the odds at 0.2%. I put the odds of an evacuation occurring during the convention in the current situation at 3%.

Figure 2. Morning satellite image of Tropical Depression Ten.

Tropical Depression Ten forms in the Eastern AtlanticThe large tropical wave in the Central Atlantic, midway between Africa and the Lesser Antilles Islands, has become Tropical Depression Ten. The depression has an impressive amount of spin and heavy thunderstorm activity, as seen on visible satellite loops, and should be Tropical Storm Joyce by Thursday. None of the models show that TD 10 will be a threat to the Lesser Antilles Islands.

Which model should you trust?Wunderground provides a web page with computer model forecasts for many of the best-performing models used to predict hurricane tracks. So which is the best? Well, the best forecasts are made by combining the forecasts from three or more models into a "consensus" forecast. Over the past decade, NHC has greatly improved their forecasts by relying on consensus forecast models made using various combinations of the GFS, GFDL, NOGAPS, UKMET, HWRF, and ECMWF models. If you average together the track forecasts from these models, the NHC official forecast will rarely depart much from it, and the NHC forecast has been hard to beat over the past few years. The single best-performing model over the past two years has been the ECMWF (European Center model). The GFS model has been a close second. You can view 7-day ECMWF and 16-day GFS forecasts on our wundermap with the model layer turned on. Ten-day ECMWF forecasts are available from the ECMWF web site. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from the other major models. As seen in Figure 3, the HWRF and GFDL were well behind the ECMWF and GFS in forecast accuracy in 2011, but were still respectable. The BAMM model did very well at 4 and 5-day forecasts. The UKMET, NOGAPS, and CMC models did quite poorly compared to the ECMWF, GFS, GFDL, and HWRF. For those interested in learning more about the models, NOAA has a great training video (updated for 2011.)

Southern COC is winning just in convection. Northern one has a lower pressure but will probably dissipate if this keeps on. Wouldn't be the first time you see a COC rolling out without any convection at all. Should throw a wrench in the models... at least this will keep it 170+ miles south of PR, but poor Haiti =/. More time on water.

Quoting sar2401:Wholesale gasoline is now up over 4 cents today, and almost seven cents since last night. Based on the CME model, there are a lot more bets being placed on Central Gulf Coast hit for Isaac. OTOH, OJ futures are near limit up for September, so the CME model also still forecasts a Florida hit. It looks like the CME model is about as reliable as the rest right now. Friday should be interesting, as traders square up positions for the weekend.

The CME model is the poster child of wishcasting.

Every week there is a major "new" cause to increase prices... and that "new" cause is repeated every few months. Yet there is rarely anything to drive the price downward, other than the price itself. I think its pretty easy to discount the CME model for being terribly biased.

Hello WPB! Not liking the consistency the ECMWF is showing away from the other models. A lot of times the ECMWF catches these deviations from track before the other globals pick them up. Not sold on it though due to the very strong model support from the other global and dynamical models..

Overall trend today has been towards the Gulf somewhere. W Coast of Florida looks to be in the crosshairs for now.

Yup, seeing that also. The old CoC at 16.1n is the dying COC with a new one developing at around 15.4W. Very pronounced windshift from the east to the west. That is the main center and it is becoming dominate quickly. That confirms a lot of thoughts around here. Whenever the old CoC dies off completely we probably have intensification.

The Port Charlotte/Fort Myers area was under a Hurricane Warning 24 hours before Charley struck. The fact that people focus on the center line and ignore the warnings is not the fault of the NHC. Charlie Archive

The Port Charlotte/Fort Myers area was under a Hurricane Warning 24 hours before Charley struck. The fact that people focus on the center line and ignore the warnings is not the fault of the NHC. Charlie Archive

exactly, figure on it being close to the coast..plan and prepare for it..IF it does not come..you were prepared..if it Does come..your prepared

I just wanted to thank you all for your kindness in responding to my post about my cruise this weekend! I will continue watching it throughout today and I will also call the cruise line. Thank you all so much and I will be watching extremely closely all day today!

Wholesale gasoline is now up over 4 cents today, and almost seven cents since last night. Based on the CME model, there are a lot more bets being placed on Central Gulf Coast hit for Isaac. OTOH, OJ futures are near limit up for September, so the CME model also still forecasts a Florida hit. It looks like the CME model is about as reliable as the rest right now. Friday should be interesting, as traders square up positions for the weekend.

Hello WPB! Not liking the consistency the ECMWF is showing away from the other models. A lot of times the ECMWF catches these deviations from track before the other globals pick them up. Not sold on it though due to the very strong model support from the other global and dynamical models..

GFS consistency vs ECMWF consistency. Last time GFS caught it? Will The ECMWF get some support from the models backing up the GFS?

ALL EYES THEN TURN TO TROPICAL STORM ISAAC AND ITS POTENTIAL THREAT TO SOUTH FLORIDA THIS WEEKEND INTO EARLY NEXT WEEK. MODEL GUIDANCE IS STILL FAIRLY SPREAD OUT BY SUNDAY. THE OFFICIAL FORECAST TRACK FROM THE NATIONAL HURRICANE CENTER BRINGS ISAAC ACROSS EASTERN CUBA AND TOWARDS THE SOUTH AND SOUTHWEST COASTS OF SOUTH FLORIDA BY MONDAY MORNING AS A STRENGTHENING HURRICANE. THE 12Z GFS MODELFOLLOWS THIS TRACK VERY CLOSELY. THE GFS HAS A LARGE WEAKNESS OVERTHE EASTERN GULF TO THE BAHAMAS...WHICH WOULD ALLOW ISAAC TO TURNMORE TO THE WEST NORTHWEST AND NORTHWEST FROM CUBA TOWARDS THEGENERAL DIRECTION OF SOUTH FLORIDA. SO POSSIBLE IMPACTS TO SOUTHFLORIDA ARE HIGHLY DEPENDENT ON THE STRENGTH OF THIS RIDGE...THESTRENGTH OF ISAAC...AND HOW IT INTERACTS WITH THE TERRAIN OFHISPANIOLA AND EASTERN CUBA. IT SHOULD BE NOTED THAT TRACKFORECAST ERRORS AT DAY 5 ARE ON THE ORDER OF 225 MILES. SO IT ISSTILL TOO EARLY TO DETERMINE THE IMPACTS OF TROPICAL STORM ISAACTO SOUTH FLORIDA OR THE SURROUNDING MARINE AREAS. HOWEVER...THEREIS A POTENTIAL THAT SOUTH FLORIDA AND/OR THE SURROUNDING MARINEAREAS COULD SEE TROPICAL STORM OR HURRICANE CONDITIONS BEGINNINGON SUNDAY. SO NOW IS A GOOD TIME TO REVIEW INDIVIDUAL TROPICALCYCLONE PREPAREDNESS PLANS.

Quoting MIFlyer:The fact that the track of the GFS has moved further west for the 1400 EDT data makes me feel better, since I live near the Cape.

But this storm reminds me so much of Charley in 2004. The TPC prediction on that one was an epic failure. I worked with a meterologist then and the day before the storm struck he said it looked like to him it would come more to the east - based not on models but on observed conditions.

When the storm did turn at Ft Myers the TPC prediction for that day was for the track to move still further west. And when it made that turn the TPC seemed to go catatonic at that point. Local meterologists took over and told us quite accurately what it was going to do.

Can anyone knowledgable here compare the 2004 model accuracy to that of today?

The Port Charlotte/Fort Myers area was under a Hurricane Warning 24 hours before Charley struck. The fact that people focus on the center line and ignore the warnings is not the fault of the NHC. Charlie Archive

Could be Isaac's movement at 21mph is causing big problems. I commented earlier that the SE storms were being pulled into the circulation, but that was wrong. May be the opposite, Isaac is moving too quickly to organize his large body of storms. Could possibly explain the reformation of the center also. Last two storms in the area this year suffered from far more inhibiting conditions, but forward speed was certainly one of them.

Pretty much have to go with a blend of both GFS and EURO right now with Issac in days 4 and 5.. somewhere from the south coast of Cuba to north Coast of Cuba.. heading NW on Monday into the extreme eastern GOM.. but there are alot of things changing across the southern U.S. by Day 5

True. Just posted because someone said the Euro had shifted east on the new run.

Also further north/east in medium range, but landfall still looks like Louisiana.

Hello WPB! Not liking the consistency the ECMWF is showing away from the other models. A lot of times the ECMWF catches these deviations from track before the other globals pick them up. Not sold on it though due to the very strong model support from the other global and dynamical models..

This is why I didn't want to give you a definite answer. Looks very suspicious right now. No true westerlies with the northern-most spin, but the southern-most spin has a higher pressure. It might be lowering, however, due to the fact that it is surrounded by more thunderstorms. Interesting.

Very interesting. If it wasn't for the RECON and radar, this looks like a well developed TS on infrared imagery.

Quoting LargoFl:ok seems my local mets are thinking issac IS going to come up the west coast of florida,they have been pretty good so far..im going to listen to them the next few days or so and prepare for a storm,not going to get caught off guard this time for sure..